In most bicycles, the pedal crank and driving sprocket wheel set are attached to a cross shaft that is journaled in ball bearings received within a bottom bracket shell attached to the down tube, seat tube and chain stays of the bicycle frame. Ordinary ball bearings carry loads that act perpendicular to the axis of rotation of the shaft. Accordingly, torques applied to a shaft mounted in them in a direction tending to skew the shaft resolve as forces on the bearings that are a function of the distance between them. The diameter of the bearings is of little consequence to the bearing loads due to torques on the supported member tending to skew the member. In conventional bottom bracket designs, therefore, it is suitable to have a bottom bracket shell of a relatively small diameter. The axial length of the shell and the axial spacing of the bearings are limited by the lateral offset of the driving sprocket wheel set and is generally small. Accordingly, bearing loads due to lateral torques (lateral with respect to the shaft axis) applied to the bottom bracket shaft due to leg forces on the crank arms and the pulling force on the drive chain are high.
The bearings in conventional bottom bracket assemblies are usually centered on the vertical-longitudinal center plane of the bicycle, while the driving sprocket wheel set is offset from that plane so as to be in line with the driven sprocket wheel set attached to the rear wheel. The chain pulling force, therefore, always produces a torque on the bottom bracket shaft tending to skew the shaft. During part of each rotation of the crank, the leg forces on the crank act produce lateral torques in the same direction as the chain pull force. The bottom bracket bearing loads dues to lateral torques are, therefore, large and reduce efficiency and cause wear.